Journey Into Space: Gravity, Orbits, and Collisions
Space Astronomy and Space
E-mail a Friend
Dr. Jack Kaye A. 

The Earth is surrounded by layers of gases we call air, or our atmosphere. Our atmosphere keeps a portion of the sun's radiation from reaching the Earth's surface, which prevents our planet from becoming boiling hot during the day. At night, it keeps heat from escaping too quickly. This protects the Earth's surface from extreme cold. Because the atmosphere moderates temperatures and screens out dangerous radiation, the Earth is able to support life.

The atmosphere also helps make the Earth a more pleasant place to live. Air carries sound waves, which let us hear voices and music. The molecules, particles, and water droplets in the air can scatter sunlight, giving rise to blue skies, red sunsets, and rainbows.

What Our Atmosphere Is Made of

Air is a mixture of colorless gases, water vapor, and dust particles. About79 percent of air is nitrogen; about 21 percent is oxygen. People need oxygen to breathe and to turn food into energy. Another gas in the atmosphere is carbon dioxide. Although it is present in very small amounts, it is vital. Green plants use carbon dioxide when they make their food.

Air contains traces of still other gases. Helium and hydrogen are found in trace amounts, as are argon, krypton, neon, and xenon. Some gases, such as methane and nitrous oxide, rise from the Earth's surface largely as a result of biological processes in plants and animals. Others, such as carbon monoxide and chlorofluorocarbons (CFC's), enter the atmosphere as a result of human activities, especially industry. Molecules such as ozone are formed in the atmosphere usually as a result of chemical reactions caused by sunlight acting on the other molecules there.

Water vapor, water in a gaseous form, is found mostly in the lowest few miles of the atmosphere, but it can be found higher up as well. It rises from bodies of water such as oceans, lakes, and rivers, and it evaporates from the Earth's surface. Clouds and fog can form from this vapor. The water in clouds can evaporate back into the atmosphere or fall to Earth as rain, sleet, or snow.

The lower atmosphere is filled with countless specks of dust tiny particles of matter from soil, fires, plants, salt spray, volcanoes, or meteors, most too small to be seen.

Air Has Mass and Exerts Pressure

Although it may feel as though air does not have mass, it does. A cubic centimeter of dry air has a mass of about 0.00118 grams when it is at sea level and at a temperature of 77F (25C). If we could measure the mass of all of the molecules in the atmosphere, the total would be staggering, about 5.7 quadrillion tons.

The air presses down on us and against us from all sides. About a ton of air is pressing against you now. You are not aware of this because pressure within your body balances the pressure of the air outside of it.

Air pressure is 14.7 pounds per square inch (1.036 kilograms per square centimeter) at sea level. It is greatest there because that is the bottom of the atmosphere. The higher you go, the thinner the air becomes. For example, at 10 miles (16 kilometers) above sea level, the density of the air is only about one-tenth of the density at the Earth's surface.

Layers of Atmosphere

Our atmosphere is made up of the troposphere, the stratosphere, the mesosphere, the thermosphere, and the exosphere.

The Troposphere

This lowest layer of the atmosphere, where we live, is the layer we know best. Like the other layers it varies in size. The troposphere extends from sea level to an altitude of almost 12 miles (19kilometers) above the equator, but only about 5 miles (8 kilometers) above the North and South poles.

Most people live less than 1 mile (1.6 kilometers) above sea level. But people can go about 31/2 miles (5.6 kilometers) above the Earth's surface before they must use pressure suits and oxygen masks.

Most weather takes shape in the troposphere. Winds pick up water vapor from which clouds and rain form. Air currents move up and down, while winds blow north, south, east, and west, carrying warm or cold air.

Instruments carried aloft in balloons and on satellites have proven that the temperature in the troposphere drops steadily as one goes higher. It drops about 3.5 to 5.5F for each 1,000 feet (2 to 3C for each 300 meters).At the top of the troposphere the temperature usually approaches 70F(56C), but it may fall as low as 117F (82C).This process slows near the top of the troposphere.

The area in which the temperature stops changing as you rise is called the tropopause, the boundary between the troposphere and the stratosphere. Winds reach their greatest force in the tropopause. Most of the winds called jet streams are here, traveling up to 200 miles (320 kilometers) an hour.

The Stratosphere

The second layer of air is the stratosphere. Temperatures in the stratosphere increase with altitude from the lower boundary(the tropopause) to the upper boundary (the stratopause). The stratopause is about 30 miles (50 kilometers) above sea level. The temperature there is typically about 32F (0C). The increase in temperature is caused by a layer of ozone. This gas absorbs most of the ultraviolet radiation that comes from the sun and changes it to heat energy, which is transmitted to other gas molecules in the stratosphere.

Ordinarily only between one and ten of every million molecules in the stratosphere are ozone molecules, but these molecules are very important. Ozone absorbs much of the sun's ultraviolet radiation, which can cause skin cancers and cataracts in people, so that only small amounts reach Earth.

The protective layer of ozone is endangered by certain pollutants, especially CFC's. In fact, the ozone layer has developed a hole, centered over Antarctica, that scientists are carefully monitoring.

The stratosphere is very stable, with little or no upward or downward movement of air. The air in the stratosphere is very dry, with only two to six molecules of water for every million molecules of air.

The Mesosphere

Above the stratosphere, the air becomes even thinner, and the temperature again falls. Beginning at a height of about 30 miles (50 kilometers) and extending up to about 50 miles (80 kilometers) is the mesosphere. At the top of the mesosphere, temperatures may be lower than 103F (75C).

The Thermosphere and the Exosphere

The thermosphere reaches a height of perhaps 250 miles (400 kilometers) above sea level. At the bottom of this layer, temperatures are below freezing. At the top, they exceed 2200F (1200C).

Atoms and molecules of gas in the very thin air of the thermosphere are bombarded by radiation from the sun. They are broken into smaller, electrically charged particles called ions in a region referred to as the ionosphere. Here electric currents can flow, as they do in a neon tube or fluorescent light.

The ionosphere reflects some radio waves back to Earth. This allows radio communication between distant places on Earth. For example, radio waves from North America bounce off the ionosphere and reach Africa. Without the bounce, the waves would simply fly into space.

The exosphere, the outermost layer of the atmosphere, continues out into space until it eventually merges with the atmosphere of the sun. The atmosphere here is extremely thin. Atoms and molecules travel so rapidly, particularly at the upper levels, that they regularly escape the Earth's gravitation and become part of the gases in space.

Changes in the Atmosphere

The atmosphere has remained fairly stable for many millions of years, but its temperature and composition change with time. Natural occurrences can cause changes in the composition of the atmosphere, for example, when ashes and hot gases are thrown into the atmosphere during volcanic eruptions.

People also cause changes. Agriculture emits gases, as does industry, which also emits trace metals and CFC's. The burning of fossil fuels such as coal and oil releases carbon dioxide, a greenhouse gas that adds to global warming. To study atmospheric changes, scientists collect data with research balloons, space shuttles, and space satellites. Scientists interpret the data with sophisticated computers.

Shop for the best in science books, kits, and more.